Ba-Cu-Si Clathrates: Phase Equilibria and Crystal Chemistry X. YAN, 1,2 G. GIESTER, 3 E. BAUER, 1 P. ROGL, 2 and S. PASCHEN 1,4 1.—Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10, 1040 Vienna, Austria. 2.—Institute of Physical Chemistry, University of Vienna, Wa ¨ hringerstr. 42, 1090 Vienna, Austria. 3.—Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, 1090 Vienna, Austria. 4.—e-mail: paschen@ifp.tuwien.ac.at The formation and crystal chemistry of ternary clathrates in the Ba-Cu-Si system were investigated on a series of compounds Ba 8 Cu x Si 46x (3 £ x £ 8). The phase diagram around the clathrate phase was constructed at 800°C, revealing a homogeneity range from Ba 8 Cu 3.4 Si 42.6 to Ba 8 Cu 4.8 Si 41.2 . Struc- tural investigations confirmed that the clathrates in this system crystallize with cubic primitive symmetry, in the type I clathrate structure (space group Pm  3n). Single-crystal x-ray diffraction indicates that the Cu atoms partially substitute for Si atoms on the 6d site; no vacancies are observed. Key words: Clathrate, phase equilibria, crystal chemistry INTRODUCTION Thermoelectric (TE) materials have in recent years attracted significant attention due to their possible application as power generators to convert waste heat into electricity. 1,2 The efficiency of TE materials can be ranked by a figure of merit, ZT, which is defined as ZT = TS 2 /(qj), where S is the Seebeck coefficient, q is the electrical resistivity, j is the thermal conductivity, and T is the absolute temperature. For simple semiconductors an enhancement of the TE efficiency can be achieved by properly tuning the charge carrier concentration via substitutions or doping. Further approaches are dimensionality reduction to improve the Seebeck coefficient, 3,4 realization of the phonon-glass elec- tron-crystal (PGEC) concept to increase the electri- cal to thermal conductivity ratio, 5 introduction of nanometer-size grains or other nanostructures in a matrix phase, 6–8 etc. Since the parameters defining ZT are interrelated, 9 optimization of ZT has turned out to be extremely difficult. Due to their particular structural characteristic, Ge- and Si-based clathrates have been considered as a class with potential for TE applications. The fullerene-like face-sharing polyhedral Ge(Si)-cages form a three-dimensional sp 3 -hybridized covalent network. The cages can be filled by large, heavy atoms such as alkali or alkaline-earth elements, which can act as rattlers to significantly reduce the thermal conductivity. 10–12 Efforts to optimize the thermoelectric performance of clathrates have focused both on the guest atoms and on the host structure. Substitution of host atoms by transition metal or group III elements was shown to: (1) stabi- lize the structure 13–16 via fulfilling the ‘‘Zintl concept,’’ 17 (2) modify the density of states at the Fermi level, which is closely related to the transport properties, 14,18 and (3) reduce the thermal conduc- tivity. 19 Herein, we report on clathrates in the Ba-Cu-Si system. Previous investigations have mainly focused on the influence of Cu substitutions on supercon- ductivity properties, 18 on site preferences, 13,20 and on theoretical calculation of the electronic struc- ture. 18,21–23 Little experimental information is available on thermoelectric properties. 14 Since in some cases the impurity phases play an important role in the transport properties, 24,25 determination of phase relations to the clathrate at certain tempera- tures is crucial. Therefore, the present research work attempts to (1) elucidate the formation and homoge- neity region of the clathrate, (2) find its phase rela- tions at 800°C to the surrounding phases, and (3) present a description of the crystal chemistry for the clathrate phase in this system. EXPERIMENTAL PROCEDURES A series of alloys with nominal compositions Ba 8 Cu x Si 46x (x = 3, 4, 5, 6, 7, 8) (denoted by S01 (Received July 9, 2009; accepted April 21, 2010; published online May 22, 2010) Journal of ELECTRONIC MATERIALS, Vol. 39, No. 9, 2010 DOI: 10.1007/s11664-010-1253-x Ó 2010 TMS 1634